In general terms "discriminisation by way of victimisation" in a racial (or gender) context occurs if someone is treated less favourably than would otherwise have been the case because he has exercised, or intends to exercise, rights which he has under the Race Relations Act 1976 (or under the Sex Discrimination Act 1975).
Treating a person badly because he has taken steps to stop unlawful racial discrimination is not normally itself racial discrimination, but may be just as reprehensible. Subject to certain conditions, it may be unlawful as "discrimination by way of victimisation ".
See also Sex discrimination/victimisation and/or Bullying at work